On Mon, Jan 07, 2013 at 02:31:21PM -0800, H. Peter Anvin wrote: > On 01/07/2013 07:14 AM, Rik van Riel wrote: > > On 01/07/2013 03:12 AM, Shaohua Li wrote: > >> > >> We use access bit to age a page at page reclaim. When clearing pte > >> access bit, > >> we could skip tlb flush for the virtual address. The side effect is if > >> the pte > >> is in tlb and pte access bit is unset, when cpu access the page again, > >> cpu will > >> not set pte's access bit. So next time page reclaim can reclaim hot pages > >> wrongly, but this doesn't corrupt anything. And according to intel > >> manual, tlb > >> has less than 1k entries, which coverers < 4M memory. In today's system, > >> several giga byte memory is normal. After page reclaim clears pte > >> access bit > >> and before cpu access the page again, it's quite unlikely this page's > >> pte is > >> still in TLB. Skiping the tlb flush for this case sounds ok to me. > > > > Agreed. In current systems, it can take a minute to write > > all of memory to disk, while context switch (natural TLB > > flush) times are in the dozens-of-millisecond timeframes. > > > > I'm confused. We used to do this since time immemorial, so if we aren't > doing that now, that meant something changed somewhere along the line. > It would be good to figure out if that was an intentional change or > accidental. I searched a little bit, the change (doing TLB flush to clear access bit) is made between 2.6.7 - 2.6.8, I can't find the changelog, but I found a patch: http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.7-rc2/2.6.7-rc2-mm2/broken-out/mm-flush-tlb-when-clearing-young.patch The changelog declaims this is for arm/ppc/ppc64. Thanks, Shaohua -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>